The present invention relates generally to measurement while drilling (MWD) systems and more particularly, to an optical telemetry system for facilitating the transmission of downhole drill bit operating parameters to the surface during ongoing drilling operations.
The advantages of obtaining downhole data during drilling operations are well known. The primary focus of MWD systems has been to collect data so as to permit more economical and efficient operation of a drill bit and hence, the drilling operation itself. In the quest to obtain such information in real-time, a variety of telemetry systems have been developed to communicate downhole information regarding drill bit performance to the surface whereby such information can be utilized to control the drilling process as described by Dailey in U.S. Pat. No. 5, 160,925. Such prior telemetry systems have employed one or more of the following communications schemes: 1) mud pressure pulsation, 2) hard-wired connections, 3) acoustic waves, and 4) electromagnetic waves.
In mud pressure pulse systems, the drilling mud pressure in the drill string is modulated. The pressure pulse travels up the mud column at or near the velocity of sound in the mud. The rate of transmission of data, however, is relatively slow due to pulse spreading, modulation rates and other disruptive forces, such as ambient noise in the drill string.
Hard-wired connectors have also been proposed to provide a hard-wired connection from the drill bit to the surface. There are a number of advantages to using hard-wired systems e.g. the ability to transmit data at a high rate, to send power downhole; and the capability of two-way communications.
The transmission of acoustic signals through a drill pipe offers another possibility for communication of downhole information. In such system, an acoustic generator is located downhole near or in the drill collar. However, a large amount of power is required downhole to generate an acoustic signal with sufficient energy to be detected at the surface.
The last major technique for transmission of downhole information is the use of electromagnetic (EM) waves through the drill pipe. In this system, downhole data is input to an antenna positioned downhole in a drill collar. Typically, a large pickup antenna is located around the drill fig at the surface to receive the EM signal.
Thus, while the advantages of obtaining operating parameters of a drill bit during ongoing drilling operations are well known, to date no one has successfully developed a telemetry system for communicating the operating parameters of a drill bit used as part of coring operations to obtain core samples of the earth's subterranean formations. The operating parameters of the drill bit during coring operations are especially critical because such things as drill bit temperature can substantially and adversely affect the material characteristics of the core sample being obtained. Because of the stationary nature of the core catcher barrel and its retrieval line relative to the rotary motion of the drill bit, positioning of a telemetry system therewith can be extremely difficult. Moreover, placement of the telemetry system must not interfere with the wire line system used to retrieve core samples from the borehole.
Accordingly, the present invention provides a novel telemetry system for optically communicating operating parameters for a drill bit during ongoing drilling operations, especially coring operations. The telemetry system is packaged in a fully "floating" housing which makes no direct electrical contact with either the drill pipe or the core sample retrieval wire line.